As operating temperatures of gas turbine engines increase to achieve improved fuel efficiency, advanced coatings are applied to various components to enhance thermal barrier protection and aerodynamic performance. The component substrate is usually a polymer, a ceramic, or a metal, such as a super alloy. Current coatings used on components in gas turbine hot sections, such as blades, nozzles, combustors, and transition pieces, generally belong to one of two classes: diffusion coatings and overlay coatings. Diffusion coatings are generally formed of aluminide-type alloys, such as nickel-aluminide, platinum-aluminide, or nickel-platinum-aluminide. Diffusion coatings are formed by depositing the coating onto the substrate and reacting the coating with the underlying substrate by high temperature diffusion. In contrast, overlay coatings are generally deposited intact, without reaction with the underlying substrate. Overlay coatings typically have the composition MCrAl(X), where M is an element from the group consisting of Ni, Co, Fe, and combinations thereof, and X is an element from the group consisting of Y, Ta, Si, Hf, Ti, Zr, B, C, and combinations thereof.
It has become commonplace to repair or refurbish gas turbine components, particularly airfoils, and return those components to service. During repair, protective coatings and oxidation deposits are removed as much as practicable to allow for inspection and repair of the underlying substrate. Various methods are known in the art for chemically cleaning the substrate. For example, U.S. Pat. Nos. 6,833,328; 6,863,738; and 7,935,642, all assigned to the same assignee as the present application, disclose various aqueous solutions containing phosphoric acid, nitric acid, sulfuric acid, hydrochloric acid, hydrofluoric acid, hydrobromic acid, hydriodic acid, acetic acid, perchloric acid, phosphorous acid, phosphinic acid, alkyl sulfonic acids, and mixtures and precursors thereof to chemically clean substrates. Relative motion between the aqueous solution and the substrate permits enhanced contact between the aqueous solution and the substrate. In some embodiments, for example, the aqueous solution may be continuously sprayed onto the substrate using various types of spray guns. Alternatively, the aqueous solution may be poured over the substrate and/or the substrate may be submerged in an acid bath containing the aqueous solution. Immersion time and bath temperature will depend on many factors, such as the type of coating being removed and the acid (or acids) included in the acid bath solution.
The acid bath solution removes the protective coatings and oxidation deposits on the substrate by dissolution reactions that produce metal salts of the constituent acids. The dissolution reactions and accompanying evaporation of the volatile acids gradually deplete the concentration and effectiveness of the acid bath solution, increasing the cycle time for cleaning the substrate and/or disposal costs associated with disposing the depleted aqueous solution. As a result, a method for refreshing, replenishing, or rejuvenating the acid bath solution may be useful to reducing the cycle time, extending the effectiveness of the aqueous solution, and/or reducing associated disposal costs.